PERSPECTIVES IN EVOLUTION — RITCHIE 261 



It is natural that the degree of change involved in the transforma- 

 tion of those species cannot be compared with the generic changes in 

 the horse series, reckoned in their millions of years; and yet it is 

 plain that the comparatively slight changes shown by the endemic 

 species of the Scottish mainland and the islands, as well as many less 

 marked changes seen in the geographical races of Scottish birds and 

 mammals, while of postglacial origin, have had available for their 

 development a range of years far exceeding the span of hmnan 

 observation or tradition. 



If this time factor is a necessary element in the evolution and 

 establishment of species in nature, doubt is thrown upon the validity 

 of arguments concerning evolution based upon laboratory experiments 

 in which intensification of means produces rapid change. There is 

 no reason why the reaction of an organism under such exaggerated 

 stimuli should be the same as that produced by minimal influences 

 of the same nature over an exceedingly long time. Even in inor- 

 ganic nature the reaction of inanimate environment may difl'er 

 according to the time element. A new stream makes its way along 

 fissures and weaknesses in the substratum on which it flows, and this 

 first course may determine the meanderings of the stream through 

 long ages. But concentrate the flow of a year or several years in a 

 cloudburst which falls at the source of the stream, and the track 

 made by the torrent, cutting across obstacles, may bear no resemblance 

 to the age-worn course, except that it runs downhill. The sensitive 

 organism delicately adjusted to a particular enviromnent is less likely 

 than inorganic environment to give a "natural" answer under con- 

 centrated compulsion. 



ADVENT OF MAN AND EVOLUTION 



The lengthening of the time perspective of life upon the earth 

 adds new insignificance to the span of man's tenancy of the world 

 and new impressiveness to the part he has played as an agent in 

 evolutionary processes. Man of our own genus, beginning in the 

 early Pleistocene period, has probably less than a million years 

 behind him, but the species of man now dominant in the world 

 {Homo sapiens) appeared only at the close of the Wiirm glacial 

 stage, no longer than 25,000 to 40,000 years ago. Yet even this rela- 

 tively short space of time exceeds man's span as an effective agent 

 in world change, for in spite of the arts he developed in early post- 

 glacial times he remained practically submerged in the fauna, having 

 little more influence upon his enviromnent than the beasts with which 

 he shared it. 



Man walked with beast, joint tenant of the shade; 

 The same his table, and the same his bed. 



— Pope. 



